It is known in certain engines to mount an oil pump on the front end of an engine offset from the axis of the crankshaft and to drive the oil pump by means of a gear on the crankshaft meshing with a gear on the oil pump that is offset from the axis of the crankshaft, so that the oil pump is driven only indirectly by the engine crankshaft.
It is desirable to be able, in different variants of the same engine, to substitute for the gear driven oil pump a concentric oil pump that is driven directly from the crankshaft. This, however, presents problems because the pumping elements of the directly driven oil pump include a sintered iron rotor that must be fitted over the nose of the crankshaft and coupled for rotation with it. The coupling mechanism on the crankshaft must be sufficiently simple to be incorporated into the manufacturing process of the crankshaft without adding considerably to the manufacturing costs. This consideration precludes the machining of splines or gear teeth on the outer surface of the crankshaft. Furthermore, the coupling mechanism must have a geometric shape that will drive the sintered iron rotor without causing damage or wear and this precludes the use of a Woodruff key. A further consideration is that it is necessary to ensure that the crankshaft should remain usable with a gear driven oil pump.